In the field of comparative spectrophotometry, it is well known to split the optical beam of a spectrophotometer into two separate optical paths, and to compare the intensities of the beams after they have traversed separate samples. Because of limitations imposed by manufacturing tolerances and non-uniformity of optical materials used in such instruments, a variable differential will exist between the optical attenuations of the two paths, even with no samples present, and it is therefore necessary to compensate for this differential in order to maintain the accuracy of the instrument throughout its range.
One means of achieving this compensation is to derive an electrical compensating signal from a potentiometer which is adjusted by means of a cam which moves synchronously with the grating drive shaft. The cam is arranged to have an adjustable profile, which is set during manufacture to cause the signal from the potentiometer to be proportional to the attenuation differential throughout the range of adjustment of the instrument. Subsequent adjustment may be necessary during the life of the instrument to compensate for ageing of optical components. In this prior system, the cam has a cam surface which is constituted by a closed loop of arcuate strips, lying approximately on a cylindrical locus, and a cam follower engages the edges of these strips. Each strip is articulated at its ends to the immediately adjacent strips, by means of screws which pass through the strips and are received in corresponding posts carried on a base plate. The profile presented by the strips is thus adjustable by moving the points of articulation along the posts. In order to accommodate changes in length of the loop of strips as the individual strips are inclined more or less to a diametric plane of the cylindrical locus, each strip is drilled at one end with a circular hole, and slotted at its other end, to permit lost motion at each articulation.
It has been found that this type of cam is troublesome to set up, and that discontinuities in response can occur, especially adjacent the articulation points.